Last year, we attempted to make elderflower champagne, using only the wild yeast present in the flowers. It did bubble and produce 'strength', but it also gave our brew the consistency of mucus...
A quick survey of self-suffiicency forums seems to indicate that not adding shop-bought yeast results in gloopy champagne.
We'll be testing this hypothesis this year (with baker's yeast), and would welcome any comment on the topic!
Roll on elderflowers!